Developing a silver halide emulsion having higher sensitivity with the same size is being pursued in the field of silver halide photographic materials. High-temperature rapid processing has rapidly spread in the processing stage of photographic materials in recent years, and processing time has been greatly shortened in processing various silver halide materials by using automatic processors. Accordingly, a silver halide emulsion and a silver halide photographic material having an excellent development rate are desired.
U.S. Pat. Nos. 4,439,520 and 4,425,425 disclose methods using tabular grains to meet the above demands. Further, JP-A-63-305343 (the term "JP-A", as used herein, means an "unexamined published Japanese patent application") and JP-A-1-77047 disclose methods wherein the rate of development and the ratio of sensitivity/fog are improved by controlling the development initiating point of silver halide grains having the (111) face to the apex of the grain and/or the edge thereof and areas in the vicinity thereof. These known methods are excellent techniques and have a high utility value. However, it is desired to further shorten the processing speed.
The present invention relates to a method wherein thiocyanates are positively deposited on the silver halide grains and incorporated in the silver halide photographic materials to further increase the sensitivity and to further improve the rate of development. As a method for using the thiocyanates, U.S. Pat. Nos. 2,222,264 and 3,320,069, JP-A-62-18538 and JP-A-64-37545 disclose methods wherein grains having high sensitivity can be obtained by allowing thiocyanates to exist during the formation of the grains. However, the grain disclosed in these patent specifications are silver chlorobromide grains having a silver iodide content of at least 1 mol% or a silver chloride content of at least 20%. The present inventors have made studies and found that when the silver iodide content is high, the improvement in development rate is poor and when silver chlorobromide has a silver chloride content of not lower than 20%, a problem is raised in regard to the sensitivity and color sensitization by sensitizing dyes.
U.S. Pat. No. 4,332,887 discloses a method wherein thiocyanates are used as solvents for silver halide to obtain grains having a preferred form. However, the intent of the method disclosed in U.S. Pat. No. 4,332,887 is to remove the solvents after the formation of the grains, that is, after the solvents become useless. Thus, the effect of the present invention cannot be exhibited. JP-A-59-162540 discloses a method wherein silver thiocyanate is formed on the edges of host grains of silver halide such as silver bromide or silver iodobromide grains to increase sensitivity. However, this method is disclosed with the intention of forming silver thiocyanate by limiting the site of the epitaxial growth on the host grains of the silver halide. Accordingly, the deposited amount of the formed silver thiocyanate in the Examples, by which sensitivity can be increased, is 5.times.10.sup.-2 mol per mol of silver halide.
The present invention is different from JP-A-59-162540 in that epitaxial formation by thiocyanate can not be confirmed through an electron microscope. When the thiocyanate was deposited in an amount of as large as 5.times.10.sup.-2 mol per mol of silver halide on the silver halide grains as in the background art, the resulting photographic material greatly suffered from an increase in photographic fog and desensitization with the passage of time and did not exhibit practical performance.